In recent years, from the viewpoint of approaching to environmental issues such as reduction of carbon dioxide emission, there arises an urgent need that automobiles, construction machines and agricultural machines consume less energy, i.e., are reduced in fuel-consumption thereof. Particularly, there is a growing demand that their devices such as engines, transmissions, final reduction gear units, compressors and hydraulic equipment contribute energy saving. Therefore, the lubricating oils used in these devices are demanded to be less in stirring resistance and frictional resistance than ever before.
Lowering the viscosity of a lubricating oil may be an example as a means of improving the fuel economy performance of a transmission and final reduction gear unit contributive to improve fuel economy performance. For example, an automobile automatic transmission or continuously variable transmission has a torque converter, a wet clutch, a gear bearing mechanism, an oil pump and a hydraulic control system, while a manual transmission or final reduction gear unit has a gear bearing mechanism. Lowering the viscosity of a lubricating oil to be used in such transmissions can reduce the stirring and frictional resistances of the torque converter, wet clutch, gear bearing mechanism and oil pump and thus enhance the power transmission efficiency thereof, resulting in an improvement in the fuel economy performance of the automobile.
However, lowering the viscosity of the lubricating oil used in these transmissions causes the above-described devices and mechanisms thereof to be significantly shortened in fatigue life and may generate seizure resulting in some defects in the transmissions. Particularly when a low viscosity lubricating oil is blended with a phosphorus-based extreme pressure additive for enhancing the extreme pressure properties, the fatigue life properties extremely deteriorate. Therefore, it is generally difficult to lower the viscosity of a lubricating oil. Although sulfur-based extreme pressure additives can improve the fatigue life properties of a lubricating oil, it is generally known that the viscosity of the base oil gives a more effect on the fatigue life properties than additives under low lubricating conditions.
Examples of automobile transmission oils which can render a transmission capable of maintaining various properties such as shifting properties for a long time include those obtained by optimizing and blending synthetic and/or mineral base oils, antiwear agents, extreme pressure additives, metallic detergents, ashless dispersants, friction modifiers and viscosity index improvers as disclosed in Japanese Patent Laid-Open Publication Nos. 3-39399, 7-268375 and 2000-63869. However, any of these compositions was not aimed at improving fuel economy performance and thus high in kinematic viscosity. Any of the publications does not refer to effects on the fatigue life properties obtained by lowering the viscosity of the lubricating oils at all. Therefore, a composition which can solve the foregoing problems has not been sufficiently studied yet. Furthermore, the transmission lubricating oils are required to have excellent low temperature viscosity and oxidation stability.